The benefits of high pressure oxidation in the processing of semiconductor wafers are set out in several previous disclosures, e.g., those by Boitnott et al. in U.S. Pat. No. 5,167,717 and Tools et al., U.S. Pat. No. 5,167,716.
In the prior art, steam is usually generated by evaporating de-ionized water from a hot quartz surface in a wafer chamber. This simple method works well, but as semiconductor device geometries became smaller, it has been discovered that small particles of an amorphous silica material remain on the wafer surfaces. The particle diameters are typically in the submicron to micron range.
These particles are conjectured to be the residues after evaporation of quartz-containing water droplets entrained in the steam. Droplets are created when water boils violently. The solubility of quartz in water is fairly low and dependent on the surface structure and temperature, but it is still adequate to support the above conjecture. Potential problems that can be caused by water droplets and silicious material in steam are well known to those with experience in the steam power industry (Babcock & Wilcox, "Steam/Its Generation and Usage" 39th ed. 1975).
To avoid problems caused by silica-laden water droplets, several methods are available, some of which may be used in combination. They are as follows:
1) Boiling relatively gently so that water surface agitation and droplet formation are minimized. This may not be compatible with generating large volumetric steam flows. PA1 2) Separating the water droplets from the steam vapor (drying). Various methods of doing this are well known in the steam power industry. PA1 3) Boiling the water in a vessel that does not dissolve residues harmful to the process.
The choice of boiler materials is limited by the elevated temperatures at which water boils at high pressures, and the necessity of avoiding contamination of the processes. Teflon is a possible material but it does not have good, stable thermomechanical properties when subjected to thermal cycling. Metal boilers would require rigorous testing to prove contamination-free processing. The surface of silicon carbide is rapidly converted to quartz when exposed to water at high temperature.